4,498 research outputs found
The Paleo-ocean of Mars
A Paleo-ocean on the northern plains of Mars is proposed. The hypothetical ocean would have formed very early in Mars' history, during the early period of rapid outgassing and cratering. As the ocean froze and receded, bursting of aquifers along the shoreline would create catastrophic flooding. Analysis of soil at the two Viking landing sites, both of which occur on the floor of the hypothetical ocean, is not inconsistent with an oceanic clay rich in water soluble salts
Kinematic alpha effect in isotropic turbulence simulations
Using numerical simulations at moderate magnetic Reynolds numbers up to 220
it is shown that in the kinematic regime, isotropic helical turbulence leads to
an alpha effect and a turbulent diffusivity whose values are independent of the
magnetic Reynolds number, \Rm, provided \Rm exceeds unity. These turbulent
coefficients are also consistent with expectations from the first order
smoothing approximation. For small values of \Rm, alpha and turbulent
diffusivity are proportional to \Rm. Over finite time intervals meaningful
values of alpha and turbulent diffusivity can be obtained even when there is
small-scale dynamo action that produces strong magnetic fluctuations. This
suggests that small-scale dynamo-generated fields do not make a correlated
contribution to the mean electromotive force.Comment: Accepted for publication in MNRAS Letter
Simulations of a Magnetic Fluctuation Driven Large Scale Dynamo and Comparison with a Two-scale Model
Models of large scale (magnetohydrodynamic) dynamos (LSD) which couple large
scale field growth to total magnetic helicity evolution best predict the
saturation of LSDs seen in simulations. For the simplest so called "{\alpha}2"
LSDs in periodic boxes, the electromotive force driving LSD growth depends on
the difference between the time-integrated kinetic and current helicity
associated with fluctuations. When the system is helically kinetically forced
(KF), the growth of the large scale helical field is accompanied by growth of
small scale magnetic (and current) helicity which ultimately quench the LSD.
Here, using both simulations and theory, we study the complementary
magnetically forced(MF) case in which the system is forced with an electric
field that supplies magnetic helicity. For this MF case, the kinetic helicity
becomes the back-reactor that saturates the LSD. Simulations of both MF and KF
cases can be approximately modeled with the same equations of magnetic helicity
evolution, but with complementary initial conditions. A key difference between
KF and MF cases is that the helical large scale field in the MF case grows with
the same sign of injected magnetic helicity, whereas the large and small scale
magnetic helicities grow with opposite sign for the KF case. The MF case can
arise even when the thermal pressure is approximately smaller than the magnetic
pressure, and requires only that helical small scale magnetic fluctuations
dominate helical velocity fluctuations in LSD driving. We suggest that LSDs in
accretion discs and Babcock models of the solar dynamo are actually MF LSDs.Comment: 12 pages, 34 figure
Generation and description of a class of random processes
Generation of possibly nonstationary random process with specified autocorrelation functio
Mean Field Dynamos with Algebraic and Dynamic alpha-Quenchings
Calculations for mean field dynamo models (in both full spheres and spherical
shells), with both algebraic and dynamic --quenchings, show qualitative
as well as quantitative differences and similarities in the dynamical behaviour
of these models. We summarise and enhance recent results with extra examples.
Overall, the effect of using a dynamic appears to be complicated and
is affected by the region of parameter space examined.Comment: 6 pages, 2 postscript figures, also available at
http://www.maths.qmw.ac.uk/~eo
Disk Winds, Jets, and Outflows: Theoretical and Computational Foundations
We review advances in the theoretical and computational studies of disk
winds, jets and outflows including: the connection between accretion and jets,
the launch of jets from magnetized disks, the coupled evolution of jets and
disks, the interaction of magnetized young stellar objects with their
surrounding disks and the relevance to outflows, and finally, the link between
jet formation and gravitational collapse. We also address the predictions that
the theory makes about jet kinematics, collimation, and rotation, that have
recently been confirmed by high spatial and spectral resolution observations.
Disk winds have a universal character that may account for jets and outflows
during the formation of massive stars as well as brown dwarfs.Comment: 18 pages, 5 figures, review to appear in Protostars and Planets V, B.
Reipurth, D. Jewitt, and K. Keil (eds.), University of Arizona Press, Tucson,
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